Character Analysis
Yellow Horse is a Sioux warrior who saves Glass's life by taking him to a medicine man who heals his maggot-infested wounds. Later, Yellow Horse escorts Glass to Fort Brazeau. That's a true friend right there.
What's more, Yellow Horse proves himself to be quite perceptive regarding the complex socio-political tumult that's now striking the frontier. Just check out his internal monologue upon arriving at Fort Brazeau, for example:
Now came the fight with the Arikara and the soldiers. True, it was the Arikara that the whites sought to punish, a tribe for which he himself held no goodwill. And true, the white soldiers had been cowards and fools. He struggled to understand his unease. Taken bit by bit, none of the foreboding seemed overwhelming. Yet Yellow Horse sensed that these scattered strands came together somehow, braided in a warning that he could not yet fully perceive. (1.15.28)
Although he doesn't fully realize it, Yellow Horse is accurately predicting the future. The U.S. Army is here to stay after the battle with the Arikara, and the following century will see American civilization further encroaching on the great Western wilderness. Even worse, this new regime will not be kind to proud Sioux warriors like Yellow Horse. He might not be able to fully articulate all of this, but Yellow Horse can feel which way the wind blows.